1. Field of the Invention
The present invention generally relates to a control system for a motor, and in particular to a motor drive control system having a function of measuring frequency characteristics of the motor.
2. Description of the Prior Art
In recent years, it has been widely practiced to obtain frequency characteristics of a motor with a load mechanically connected thereto for the purposes of analyzing a mechanical resonance which may cause degradation in operation performance, analyzing response characteristics and stability of control, and the like. In this description, it is noted that the term "frequency characteristics" refers to characteristics in a graph representing a relationship of a gain or phase to a frequency. The term "frequency-response characteristics" refers to the frequency characteristics of a real motion to a drive instruction where the real motion is outputted by a control object based on the input instruction via a control gain to form a feedback control loop. The term "velocity-instruction response characteristics" refers to the frequency-response characteristics in the case where the input instruction is a velocity instruction.
Conventionally, in order to measure frequency characteristics of a motor in a motor drive control system, there has been used a construction of a feedback control loop, for example, as shown in FIG. 12. Referring to FIG. 12, a motor drive unit 1 receives an input drive control instruction signal (referred to as "input instruction signal" hereinafter) and drives a motor 3 and a load 4 under control in accordance with the input instruction signal. The load 4 is mechanically connected to the motor by a driving shaft of the motor. As an input instruction signal, a velocity instruction signal Vins or a position instruction signal Pins is generally used. In this example of FIG. 12, the conventional construction is referred to a case where a velocity instruction Vins is used.
A servo-analyzer 5, which includes therein an oscillator (not shown) for generating a sine wave signal, is connected to the motor drive unit 1 so that the sine wave signal output of the servo-analyzer 5 is applied to the motor drive unit 1, serving as the velocity instruction Vins. The servo-analyzer 5 is also connected with a velocity detector 6 attached to the motor for detecting a real rotational velocity (referred t as "real motor velocity" hereinafter) Vr of the motor 3 so that the servo-analyzer 5 receives the real motor velocity Vr detected by the velocity detector 6.
With this construction, the servo-analyzer 5 outputs a sine wave signal serving as the velocity instruction Vins. The motor drive unit 1 receives the velocity instruction Vins and drives the motor 3 and the load 4 under control in accordance with the velocity instruction Vins. Therefore, the real motor velocity Vr is in a motion of a sine wave form in accordance with the velocity instruction Vins as shown in FIG. 13.
In the wave form comparison shown in FIG. 13, the servo-analyzer 5 detects a gain which is an amplitude ratio of the velocity instruction Vins to the real motor velocity Vr and a phase difference between the velocity instruction Vins and the real motor velocity Vr. By continuing the detection of the gain and phase difference while gradually increasing the frequency of the velocity instruction Vins, the frequency characteristics in a range from the velocity instruction Vins to the real motor velocity Vr are obtained. The measurement results of the frequency characteristics are generally represented as a Bode diagram.
In this conventional construction as described above, the servo-analyzer 5 is inherently required as an instrument for measuring the frequency characteristics of the motor. Therefore, in the case where the load 4 of hardware connected to the motor is too large in size and weight to be movable, the servo-analyzer 5 must be moved to the place where the load 4 is installed. In general, however, the servo-analyzer 5 is an instrument which lacks portability, and thus the conventional drive control system is very inconvenient.
Moreover, since the frequency of the velocity instruction Vins is gradually increased from the lowest frequency to the highest frequency in a range of a desired measurement bandwidth, there has been a problem that the measurement time is disadvantageously long. Meanwhile, in order to suppress an influence of a noise and the like to enhance the measurement precision of the frequency characteristics, the amplitude of the velocity instruction Vins must be increased to some desirable degree. If the measurement time is long while the amplitude is kept large, burdens affected on the motor 3 and the load 4 increase, which may in some cases cause overheating breakdown of the motor 3 or breakdown of the hardware load 4.